What is the best fps to use if one plans to add music later?
If I were you I would take your song's tempo into consideration when choosing a frame rate. This is assuming you don't have tempo changes - if you do there's nothing you can really do since the frame rate must remain constant. 

Here's a handy BPM / FPS calculator to help you choose the best frame rate for your song.

So for example, if your song is 120BPM and your video is 30FPS you are looking at 15 frames per beat.

Keep in mind that the song tempo should not be the only criteria to consider. You should consider how this video will be delivered as well, like is it only for YouTube, or does it need to play on some specific devices, etc. Higher frame rates are usually better, but will obviously increase file size and render time. If your tempo can fit into 30 or 24 FPS, one of these will be pretty easy to work in while being hi fidelity enough for most situations. 

Finally, if you have software that will allow you to change the tempo of your song, don't rule out that option.